1. Field of the Invention
The present invention relates to a method of depositing a silicon oxide layer on a semiconductor wafer, and more particularly, to a method of depositing a silicon oxide layer on the transistors and other areas over the surface of the semiconductor wafer.
2. Description of the Prior Art
In semiconductor processing, some gases such as silane (SiH.sub.4), tetra-ethyl-ortho-silicate (TEOS) and oxygen are used in a thin film deposition process to form a dielectric layer. The chemical reaction between the gases forms a thin film on the semiconductor wafer to provide electrical insulation between devices. Because the manufacturing technology of silicon oxide (SiO.sub.x) is very well developed, a silicon oxide (SiO.sub.x) layer is most frequently used as the dielectric layer in most semiconductor wafer processes. Atmospheric pressure chemical vapor deposition (APCVD) is usually employed in the prior art method. O.sub.3 and TEOS are used as reactive gases to form the silicon oxide layer. This method is abbreviated as O.sub.3 -TEOS in the semiconductor industry. The silicon oxide layer formed by this method has good gap filling ability, so it is usually used in the inter-metal layer or inter-layer dielectric.
Please refer to FIG. 1. FIG. 1 is a sectional schematic diagram of silicon oxide layer 19 deposited on semiconductor wafer 11 according to the prior art. In the prior art method of forming silicon oxide layer 19, semiconductor wafer 11 is loaded on an APCVD machine to undergo an APCVD process by using the O.sub.3 -TEOS method. Silicon oxide layer 19 will then be formed on semiconductor wafer 11. The surfaces of each transistor on semiconductor wafer 11 are usually formed by many different materials, as shown in FIG. 1. The surface of semiconductor wafer 11 comprises silicon oxide layer 12 and polysilicon 13 which is filled inside silicon oxide layer 12. The deposition layer 19 on semiconductor wafer 11 and transistors is formed by gases mixed with TEOS and a predetermined amount of highly concentrated ozone.
However, highly concentrated ozone is very sensitive to the surfaces formed by different materials on the semiconductor wafer 10. The relative deposition rates of each material are as follows: Si &gt;SiN&gt;SiO.sub.2 formed by a thermal oxidation process &gt;SiO.sub.2 formed by a deposition process. Consequently, the deposition rate of deposition layer 19 formed on the surface of the semiconductor wafer 11 will vary with the different surface materials. Hence, if highly concentrated ozone is used to perform the deposition process, silicon oxide layer 19 will form on semiconductor wafer 11 with uneven thicknesses, that will reduce the manufacturing yield.
Therefore, for solving the above problem caused by employing highly concentrated ozone as the reactive gas in the deposition process, the semiconductor industry usually creates a buffer layer on the semiconductor wafer first, in order to reduce the sensitivity of highly concentrated ozone. Please refer to FIG. 2. FIG. 2 is a sectional schematic diagram of silicon oxide layer 17 deposited on the semiconductor wafer 10 according to the prior art. The semiconductor wafer 10 comprises a plurality of transistors 12 positioned on its surface, wherein the transistors consist of Si, SiO.sub.2 and SiN. In the prior art method, TEOS and a low concentration of ozone are injected to form the first silicon oxide layer 14 as a buffer layer on the semiconductor wafer 10. Then TEOS and highly concentrated ozone are injected to form the second silicon oxide layer 16 on the semiconductor wafer 10. The silicon oxide layer 17 consists, then, of the first silicon oxide layer 14 and the second silicon oxide layer 16, and is used as the dielectric layer.
For the silicon oxide layer 17 shown in FIG. 2, TEOS and a low concentration of ozone are injected as the reactive gases. Because a low concentration of ozone also has a low sensitivity characteristic, the first silicon oxide layer 14 will be formed on the semiconductor wafer 10 with a thin thickness. Then highly concentrated ozone and TEOS are injected to form the second silicon oxide layer 16 with a thick thickness and a uniform surface over the first silicon oxide layer 14. Because the second silicon oxide layer 16 is deposited on the surface of the first silicon oxide layer 14, which is a single material, the sensitivity of highly concentrated ozone to the semiconductor wafer 10 is uniform. Consequently, the second silicon oxide layer 16 forms as a flat surface.
However, the gap filling ability of the first silicon oxide layer 14 is very poor. Voids 18 are easily formed in trenches, or in places between devices during the deposition process. These defects can result in short circuits and reduce the reliability of the transistors on the semiconductor wafer 10, which increases production costs. Moreover, as the manufacturing technology improves, the design of line widths and line distances on the semiconductor wafer 10 decrease. The poor gap filling ability of the first silicon oxide layer 14 formed by the prior art method will cause more voids 18 on the semiconductor wafer 10 during the manufacturing process and will immensely effect the electrical performance of the semiconductor wafer 10.